(1) Field of the Invention
This invention relates to reverse flow heat-regenerative burner apparatus used for heating furnaces and the like while limiting heat losses. More particularly, the invention relates to methods and apparatuses that address problems of contamination of the heat exchange media used in such apparatuses.
(2) Description of the Related Art
Combustible fuel burners are often employed for heating furnaces intended for melting metals, such as scrap aluminum, glass and other materials. The burners require a supply of combustion air and generate a substantial volume of hot combustion gases that are directed into the furnace. Regenerative burners are designed to improve fuel efficiency by recycling heat from combustion gases exiting the furnace that would otherwise go to waste. Usually, two fuel burners are used in tandem and are associated with two fluid-porous bodies of heat-absorptive material, usually media beds made of particles of refractory material. The media beds absorb heat from, or deliver heat to, gas passing through the beds depending on the relative temperatures of the gas and the beds at the time of contact. As a first of the two burners is operated, the waste combustion gas that it generates is eventually withdrawn from the furnace and passed through a second one of the media beds to heat the media. Combustion gas, normally air, for the first burner is at the same time drawn through a first media bed where it is heated by media already hot from a prior burner cycle. After a period of time intended to maximize efficiency (often 30 seconds to three minutes), the first burner is turned off and the second burner is ignited. Combustion gas for the second burner is then drawn through the second media bed (the one previously heated by the waste gas from the first burner), and waste combustion gas from the second burner is withdrawn from the furnace and passed through the first media bed so that it is heated once again. By cycling the burners in this way, waste heat is captured to a large degree and is redirected back into the furnace so that the furnace is operated with improved efficiency. Clearly, it is possible to use more than two burners per furnace provided the waste gases from one group of burners are used to pre-heat the combustion gases from another other group by means of associated regenerative media beds, or the burners may be operated in pairs, with the pairs having different heating cycles from each other.
A disadvantage of regenerative burners used in this way is that their associated media beds often become contaminated by components in the waste gases taken from the furnace and passed through the beds. The nature of the contaminants depends on the material being heated in the furnace. For example, when the furnace is used for remelting scrap aluminum, salt fluxes are often added to facilitate the melting of the metal and to avoid undue metal oxidation. The salt fluxes partially vaporize at the furnace operating temperatures and subsequently condense in the media beds, thereby restricting gas flow through the beds and eventually blocking them. The vaporized salt fluxes initially condense as liquids but eventually solidify as they progress through the beds to cooler regions. This problem is well recognized in the prior art, and many attempts have been made to solve the problem by removing the contaminating deposits in one way or another.
United States published patent application No. 2002-0072020 (to Crane et al., published on Jun. 13, 2002) discloses a typical burner/media bed combination of the type used in metal melting furnaces. The publication discloses a convenient way for replacing an entire media bed when it has become contaminated to an unacceptable extent. The media bed thus removed is replaced with minimal delay by a spare media bed that contains new, renovated or otherwise decontaminated media.
U.S. Pat. No. 4,944,670 (to Watson, issued on Jul. 31, 1990) discloses a two-burner furnace with regeneration beds that may be used in an aluminum melting facility. The patent notes that the beds become contaminated with salts, etc., and discloses a control system that periodically allows the beds to be headed to an elevated temperature to melt the adsorbed salts which can then be collected and discarded.
U.S. Pat. No. 4,807,695 (to Ward, issued on Feb. 28, 1989) discloses a design of regenerator for use in a regenerative burner system for melting glass or non-ferrous metals. The invention provides an inlet for adding clean refractory balls (the media particles of the bed) at one end of the bed, and means for removing contaminated balls at the other end. In this way, contaminants are removed as the bed is refreshed.
U.S. Pat. No. 4,923,391 (to Gitman, issued on May 8, 1990) discloses a regenerative burner control system for aluminum melting in which provision can be made for bypassing the regenerative bed at certain stages of the heating cycle when contaminants are particularly high. Contamination of the bed can therefore be reduced.
British patent application GB 2 209 386 A (to Wills, et al., published on May 10, 1989) discloses a regenerative burner system for glass melting where provision is made for use of an intermediate cooling stage (between two regenerative beds) so that contaminants can be condensed in a duct section that is easy to clean.
British patent application GB 2 192 264 A (to Goodfellow, published on Jan. 6, 1988) discloses a regenerative burner system in which a regenerative bed is arranged in two parts in such a manner that the gas passes downwardly through the first part and then upwardly through the second part. The temperature profile is arranged so that the contaminated zone is positioned close to the lowermost layer of the first part of the bed, whereupon relatively small changes in the temperature profile result in movement of the blocking material from the first part of the bed in a molten state. The contaminants are collected and discarded.
US patent publication no. 2006/0093978 A1 of May 4, 2006 discloses a method and apparatus for cleaning a media bed by introducing a rapid flow of a decontaminating gas through the media bed from below to cause the particles to move relative to each other to dislodge solid contaminants.
While these solutions to the problem may be effective in some ways, they nevertheless have disadvantages, such as requiring expensive modification of the apparatus or frequent replacement of the media. There is therefore a need for other ways of dealing with contamination of the media that reduces gas flow through regenerative beds, particularly when the combustion gases contain condensable contaminants.